1. Field of the Invention
The present invention relates to a high efficiency audio amplifier and method thereof, and more particularly, to an audio amplifier capable of enhancing power efficiency through varying the frequency of the oscillating signal and related method.
2. Description of the Prior Art
Power amplifiers are utilized for increasing the power of signals and are usually classified as A, B, AB, C and D types. The class D amplifier adopts a Pulse-Width Modulation (PWM) technology, which adjusts duty cycles of square waves to represent input values. Also, the power amplifiers offer advantages of small size, high efficiency, and energy savings so that the class D power amplifier has widely been applied in various electronic products, such as notebooks, LCD monitors, mobile phones, and multimedia players, etc.
Please refer to FIG. 1A, which is a schematic diagram of a class D power amplifier 10 according to the prior art. The class D power amplifier 10 includes an input end 102, a reference signal end 104, a comparator 106, a gate driver circuit 108, an output stage 110, a filter 112, and a speaker 114. The input end 102 is utilized for receiving an input audio signal SIN. The reference signal end 104 is utilized for receiving a triangular reference signal ST. The comparator 106 is utilized for performing a comparison operation on the input audio signal SIN and the triangular reference signal ST to generate a PWM signal SP, and transmitting the PWM signal SP to the gate driver circuit 108. After that, the gate driver circuit 108 generates a gate drive signal SG according to the PWM signal SP for driving the output stage 110. Furthermore, the output stage 110 outputs an output signal SAP according to the gate drive signal SG. The filter 112 filters the output signal SAP to output an output audio signal SO. Finally, the output audio signal SO is transmitted to the speaker 114 for playing.
Please refer to FIG. 1B, which is a schematic diagram of the operation efficiency of the class D power amplifier 10. Compared with other types of power amplifiers, the class D power amplifier 10 is able to achieve much higher power efficiency, above 90%, for high output power situation in practice. However, as shown, in FIG. 1B, the class D power amplifier 10 can not achieve good power efficiency as well in a low output power situation. How to figure out a method for enhancing power efficiency in a low output power situation should be a concern in the progressive application design.